The air motor is an engine for rotating a main shaft by having the main shaft supported by static pressure gas bearings, and ejecting a gas such as compressed air toward an impeller (rotor blade) from a nozzle (holes and tubes), and is widely used, mounted on electric painting devices, high-precision machine tools, and similar devices. Various modifications of conventional devices have been made so as to improve rotation efficiency, and various motor configurations as concrete examples thereof are well-known (See Patent Document 1 and Patent Document 2).
FIGS. 1 and 2 illustrate a configuration of an air motor (spindle device with air turbine) mounted on an electrostatic spray gun of an electric painting device as a configuration example of such an air motor. This air motor includes a hollow main shaft 2, which extends in an approximately right circular tube form from a base to a tip (from right end to left end in FIG. 1), and an impeller 4, which is arranged on the base of the main shaft 2 concentric therewith. The impeller 4 includes a annular portion 6, which is a larger flat plate in diameter than the main shaft 2 and is positioned and fixed to the base of the main shaft 2 by a fastening member or the like, and an impeller main body 8, which is a short cylinder that is larger in diameter than the main shaft and smaller in diameter than the annular portion 6 and is fixed on an axial side (right side in FIG. 1) of the annular portion 6. Multiple turbine blades 10 are formed across the entire impeller main body 8 at equal intervals along the circumference thereof. Each of the turbine blades 10 is structured with the same form so as to have the same gradient (for example, forward tilting in normal rotative direction (right rotative direction C in FIG. 2) of the impeller 4) in the same rotative direction.
The main shaft 2 and the impeller 4 making such a structure are rotatably supported by predetermined bearings (radial static pressure gas bearings 14 and axial static pressure gas bearings 16) in a housing 12, respectively. In the structure shown in FIG. 1, a bearing main unit 18 of the radial static pressure gas bearings 14 is made of a porous material in cylindrical form, fixed at a central portion along the axis inside of the housing 12, and arranged such that the inner periphery thereof is arranged facing a central portion along the axis of the external surface of the main shaft 2 at a slight gap therefrom. An air supply channel 20, which supplies compressed air in spaces between the housing 12 and the periphery of the main shaft 2 via the bearing main unit 18, is provided inside of the housing 12 and extends to the external surfaces of the bearing main unit 18 of the radial static pressure gas bearings 14. Meanwhile, the axial static pressure gas bearings 16 are structured such that a bearing main unit 22 thereof made of a porous material is ring-shaped and has an oblong cross section, fixed to the base (right end in FIG. 1) of the housing 12, and is arranged such that an axial side (right side in FIG. 1) faces the circumference of the opposite side (left side) to the fixing side of the annular portion 6, which comprises the impeller 4, for the impeller main body 8. The air supply channel 20 extends to the external surface of the bearing main unit 22 of the axial static pressure gas bearings 16 so as to also supply compressed air to spaces from a side of the annular portion 6 of the impeller 4 via the periphery of the bearing main unit 22.
When rotatably supporting the main shaft 2 and the impeller 4 using the radial static pressure gas bearings 14 and the axial static pressure gas bearings 16, compressed air is continuously provided in the gaps between the bearing main bodies 18 and 22, the main shaft 2, and the impeller 4 (the annular portion 6) via the air supply channel 20, the radial static pressure gas bearings 14, the axial static pressure gas bearings 16, and the bearing main bodies 18 and 22. The compressed air supplied to the spaces is blown continuously on a side of the annular portion 6 and the external surface of the main shaft 2, forming a film of air in all of the spaces due to the compressed air. As a result, the main shaft 2 and the impeller 4 keep a noncontact state with the bearings 14 and 16 via the film, and are supported rotatably by the bearings 14 and 16.
Note that the compressed air continuously supplied to the spaces through the air supply channel 20 is successively exhausted to the exterior space via exhaust holes 24, which are provided within the bearing main body 18 of the radial static pressure gas bearings 14, an exhaust channel 26, which is provided within the housing 12, and spaces within the housing 12. In the case of mounting an air motor (spindle device with air turbine), which makes such a structure, on an electrostatic spray gun of an electric painting device, the impeller 4 and the main shaft 2 to which the impeller 4 is fixed should be aligned along the axis by other axial static pressure gas bearings (not illustrated in the drawing), which are additional ones to the axial static pressure gas bearings 16, rotatably supporting the opposite side (i.e., the fixing side for the impeller main body 8 (right side in FIG. 1)) to the supporting side of the annular portion 6, which is supported by the axial static pressure gas bearings 16.
Moreover, the impeller 4 is arranged in the housing 12 such that the inner periphery on the base side (right end side in FIG. 1) and the outer periphery of the impeller main body 8 may face each other all around. In other words, the base side inner periphery of the housing 12 is positioned radially outward from the impeller main unit 8.
Multiple (for example, six holes at equal intervals in the structure illustrated in FIG. 2) turbine air nozzle holes 28, which are formed at predetermined intervals along the circumference toward the periphery of the impeller main body 8, are formed on the base side of the housing 12, which is positioned radially outward from the impeller main body 8. These turbine air nozzle holes 28 are formed such that all centers thereof are positioned within a virtual plane orthogonal to a central axis of the housing 12, and tilt at the same angle with respect to the radial direction of the housing 12 (in other words, they forward-tilt in the normal rotative direction (right rotative direction C in FIG. 2) of the impeller 4.) Furthermore, these turbine air nozzle holes 28 continue to a turbine air supply channel 30, which has an opening 28u on an upstream end (compressed air (turbine air) supply source side) formed all around near the base side periphery of the housing 12, and the turbine air supply channel 30 continues to a turbine air supply opening 32, which opens to the base (right end in FIG. 1) of the housing 12 at one place along the circumference. Meanwhile, the respective turbine air nozzle holes 28 have downstream ends (turbine air spray inlets) 28d open to the base side inner periphery of the housing 12. In other words, the downstream ends (turbine air spray inlets) 28d of the respective turbine air nozzle holes 28 are formed closely facing the multiple turbine blades 10 formed on the external surface of the impeller main unit 8.
Furthermore, a brake air nozzle hole 34 is formed in the housing 12, opening to the periphery of the impeller main body 8 such that it does not overlap with the above multiple turbine air nozzle holes 28 on the base side. The brake air nozzle hole 34 is formed such that the center thereof is positioned within a virtual plane having the same central axis as the turbine air nozzle holes 28 (i.e., within a virtual plane orthogonal to the central axis of the housing 12 that is the same as those of the turbine air nozzle holes 28) and tilts at a predetermined angle (approximately the same angle as the turbine air nozzle holes 28) in the opposite direction than the turbine air nozzle holes 28 with respect to the radial direction of the housing 12 (in other words, forward-tilts in reverse rotative direction of the impeller 4 (left rotative direction A in FIG. 2)). Moreover, the brake air nozzle hole 34 has an upstream end (brake air supply source side) opening 34u continuing to a brake air supply opening 36, which opens to the base (right end in FIG. 1) of the housing 12, and a downstream end (brake air spray inlet) 34d opening on the base side inner periphery of the housing 12. In other words, the downstream end (brake air spray inlet) 34d of the brake air nozzle hole 34 is formed closely facing the multiple turbine blades 10 formed on the external surface of the impeller main body 8.
Note that a circular rotation detecting sensor 38 is arranged on the base side of the housing 12 such that the inner periphery of the bearing main unit 22 of the axial static pressure gas bearings 16 and the other axial side (left side in FIG. 1) of the impeller main body 8 may face each other with a predetermined distance therebetween. The rotation detecting sensor 38 includes a detector (right side portion in FIG. 1) capable of facing the other axial side of the impeller main body 8 and a to-be-detected unit (encoder) on the other side of the impeller main unit 8. This constitutes a sensor mechanism for detecting rotational state (rotation speed, rotative direction, and the like) of the impeller 4. With the sensor mechanism, the rotational state (rotation speed, rotative direction, and the like) of the impeller 4 is detected by detecting and measuring positional change of the to-be-detected unit (encoder) using the detector.
A magnet, for example is employed as the rotation detecting sensor 38 in the air motor illustrated in FIG. 1. This is because the axial bearing 16 is provided only on the output side of the rotary movement, as shown in FIG. 1, which may allow the main shaft 2, the impeller 4, and the impeller main body 8 to slip out to the opposite side to the output side (opposite direction than the output side of the rotary movement) of the rotary movement. Employment of the magnet for the rotation detecting sensor 38 thereby allows attraction to the main shaft 2 so as to reduce the chance of the main shaft 2, the impeller 4, and the impeller main body 8 from slipping out to the opposite side to the output side of the rotary movement. In this manner, as long as the rotation detecting sensor 38 can suppress the possibility mentioned above, function and arranging position may be appropriately selected according to purpose. For example, installation of the axial bearings 16 on either side of the impeller 4 allows a structure not employing a magnet as the rotation detecting sensor 38.
When coating using an electrostatic spray gun of an electric painting device on which the air motor (spindle device with air turbine) making such a structure is mounted, the air motor operates in the following manner.
As described above, the main shaft 2 and the impeller 4 are rotatably supported on the housing 12 by the radial static gas bearings 14 and the axial static gas bearings 16, respectively. In this state, compressed air (turbine air) is supplied to the multiple turbine air nozzle holes 28 via the turbine air supply opening 32 and the turbine air supply channel 30. The supplied compressed air (turbine air) is blown onto the multiple turbine blades 10 formed on the periphery of the impeller main unit 8 from the downstream ends (turbine air spray inlets) 28d of the respective turbine air nozzle holes 28. As a result, the turbine blades 10 are continuously depressed in their tilt direction, namely normal rotative direction (right rotative direction C in FIG. 2) of the impeller 4, rotating the impeller 4 and the main shaft 2 in the normal rotative direction at a predetermined rotation speed (e.g., several tens of thousands rpm).
A coating material is then supplied into a predetermined cup (not illustrated in the drawing) via a coating material supply-pipe (not illustrated in the drawing) inserted inside of the main axis 2 in this state. The cup is fixed to a portion of the front end (left end in FIG. 1) of the main shaft 2 that protrudes (is exposed) to the outside of the housing 12, and is negatively charged. As a result, the coating material supplied to the cup is made into ion microparticles within the cup that rotates at a high speed along with the main shaft 2.
The coating material made into ion microparticles is thrown toward a positively-charged surface to be coated utilizing electrostatic attraction and adhered on that surface. Note that the compressed air (turbine air) blown onto the respective turbine blades 10 is exhausted out into the outside space from an opening on the base side of a circular space 40 between the inner periphery on the base side of the housing 12 and the outer periphery of the impeller main body 8 via an exhaust channel (not illustrated in the drawing) connecting to the opening.
On the other hand, in the case of stopping the coating operation on the surface to be coated, supply of compressed air (turbine air) to the respective turbine air nozzle holes 28 and supply of the coating material to the cup are stopped, and compressed air (brake air) is supplied to the brake air nozzle hole 34 via the brake air supply opening 36. The supplied compressed air (brake air) is blown onto the multiple turbine blades 10 from the downstream end (turbine air spray inlet) 34d of the brake air nozzle hole 34. As a result, the turbine blades 10 are continuously depressed in the opposite direction of their tilt direction, namely reverse direction (left rotative direction A in FIG. 2) of the impeller 4, thereby imposing a negative load on the inertia rotation in the normal rotative direction of the impeller 4 and the main shaft 2 so as to halt early.
Then, once the rotation detecting sensor 38 has detected that the rotation speed of the impeller 4 and the main shaft 2 has slowed down and rotation thereof completely stops, supply of compressed air (brake air) to the brake air nozzle hole 34 is then stopped.
Note that even in this case, the compressed air (brake air) blown onto the respective turbine blades 10 is exhausted out to the outside space from the opening on the base side of the circular space 40.
However, driving force of the air motor is dependant on momentum of the jet flow from the nozzle that hits a turbine, namely momentum of the compressed air (turbine air) ejected from the downstream ends (turbine air spray inlets) 28d of the turbine air nozzle holes 28 to be blown onto the multiple turbine blades 10 that are formed on the periphery of the impeller 4 (more specifically, the impeller main body 8). The driving force (torque) of the impeller 4 sprayed with the compressed air (turbine air) at that time is calculated using the following Equation 1 (See Non-patent Document 1). Note that in Equation 1, T denotes torque of the turbine (the impeller 4), F denotes momentum (driving force) of jet flow (ejected compressed air from the turbine air nozzle holes 28) from the nozzle, R denotes radius of the turbine (the impeller 4 sprayed with the ejected compressed air) on which the jet flow impacts, m denotes mass (where mass flow rate×Δt) of the jet flow (ejected compressed air), V denotes flow velocity of the jet flow (the ejected compressed air), and Vt denotes circumferential velocity (where Vt is 2πRN and N denotes motor rotation frequency) at the region (region of the impeller 4 on which the jet flow impacts) impacted by the jet flow.[Equation 1]T=F·R=m(V−Vt)R  (1)
The flow velocity of the gas flowing into the nozzle (flow velocity of the compressed air (turbine air) immediately after being supplied to the turbine air nozzle holes 28 from the turbine air supply channel 30 via the upstream end openings 28u or inlet to the turbine air nozzle holes 28; hereafter it is referred to as inlet flow velocity) is not acoustic velocity even under choked conditions such that maximum velocity as j et flow is attained in the nozzle, and is calculated using the following Equation 2. Note that in Equation 2, ve denotes inlet flow velocity in the nozzle (the turbine air nozzle holes 28) in a choked state, a0 denotes acoustic velocity, and k denotes specific heat ratio of compressed air (turbine air).
                    [                  Equation          ⁢                                          ⁢          2                ]                                                                      v          e                =                              a            0                    ⁢                                    2                              k                +                1                                              ⁢                      (                          about              ⁢                                                          ⁢              313              ⁢                                                          ⁢              m              ⁢                              /                            ⁢              s                        )                                              (        2        )            
Moreover, mass (namely, maximum value of mass flow rate) of the jet flow (ejected compressed air) in the above choked state is calculated using the following Equation 3. Note that in Equation 3, mmax denotes mass of the jet flow (ejected compressed air) in the above choked state, ρ0 denotes density of the compressed air (turbine air) on the upstream side, and Ae denotes inlet area of the nozzle (the turbine air nozzle holes 28).
                    [                  Equation          ⁢                                          ⁢          3                ]                                                                      m                      ma            ⁢                                                  ⁢            x                          =                                            (                              2                                  k                  +                  1                                            )                                                      k                +                1                                            2                ⁢                                  (                                      k                    -                    1                                    )                                                              ⁢                      ρ            0                    ⁢                      a            0                    ⁢                      A            e                                              (        3        )            
where if specific heat ratio (k) is 1.40, isopiestic specific heat Cp is 1007 (J/kg·K), and temperature of the compressed air (turbine air) on the upstream side is T (K), the acoustic velocity (a0) is represented by the following Equation 4.[Equation 4]a0=√{square root over (cp(k−1)T)}  (4)
Furthermore, the density (ρ0) of the compressed air (turbine air) on the upstream side is calculated using the following Equation 5. Note that in Equation 5, P0 denotes pressure of the compressed air (turbine air) on the upstream side.
                    [                  Equation          ⁢                                          ⁢          5                ]                                                                      ρ          0                =                  1.293          ⁢                                          ⁢                      273.15            T                    ⁢                                    p              0                                      1.013              ×                              10                5                                                                        (        5        )            
In light of the above, in order to improve driving efficiency of the air motor, the inlet flow velocity (ve) (approximately 313 m/s) of the compressed air (turbine air) in the nozzle (the turbine air nozzle holes 28) in a choked state should be raised to the acoustic velocity (340 m/s). For example, expanding the compressed air (turbine air) using pressure drop in the compressed air by fluid friction (inner periphery of the turbine air nozzle holes 28) of the nozzle makes it possible to increase the inlet flow velocity (ve). However, even in this case, the maximum velocity is acoustic velocity (340 m/s).
Making the inlet flow velocity (ve) be the acoustic velocity through flow velocity increase is achieved in the case where length of the nozzle is set to L or greater, which is represented in Equation 6 (see Non-patent Document 2) given below when M1 is ve/a0. Note that in Equation 6, rh denotes hydraulic radius (inner radius in the case of round holes or circular tubes, cross-sectional area A in the case of square holes and square tubes, and is defined by 2×A/C in the case where circumference length is C), and cf denotes viscous friction factor of the wall (inner periphery of the turbine air nozzle holes 28) of the nozzle (holes and tubes). At that time, the viscous friction factor (cf) is given as 0.0576×Re−0.2 using the Reynolds number (Re=vD/ν) when v denotes flow velocity of compressed air, D denotes diameter (inner diameter) of the nozzle (holes and tubes), and ν denotes kinematic viscosity.
In this manner, Equation 6 holds true even when the cross-sectional shape of the nozzle (the turbine air nozzle holes 28) is other shapes than round, such as square.
                    [                  Equation          ⁢                                          ⁢          6                ]                                                            L        =                                            r              h                                      2              ⁢                              c                f                                              ⁢                      (                                                            1                  -                                      M                    1                    2                                                                    kM                  1                  2                                            +                                                                    k                    +                    1                                                        2                    ⁢                    k                                                  ⁢                                  ln                  ⁡                                      (                                                                                            (                                                      k                            +                            1                                                    )                                                ⁢                                                  M                          1                          2                                                                                            2                        +                                                                              (                                                          k                              -                              1                                                        )                                                    ⁢                                                      M                            1                            2                                                                                                                )                                                                        )                                              (        6        )            